Volume control and noise suppression system



March 30, 1937.

Filed April 30, 1935 M r Y P. Y T 8 E f N N E m MM WW T T I M T r I? e{F352 4 l 1 m LEWESQ smut 8 d I ZM LJSJEJL m w m a 4. M m w a m M w m um 1 o 2 c M w w T 0 1 w a Patented Mar. 30, 1937' UNITED STATES VOLUMECONTROL AND' NOISE SUPPRES- SION SYSTEM Lester T. Fowler, Oaklyn, N. J.,assignor to Radio Corporation of America, a

ware

corporation of Dela- Application April 30, 1935, Serial No. 19,036

Claims.

The present invention relates to a radio receiving detector-amplifiersystem and more particularlyto volume control and noise suppressioncircuits.

6 In radio receivers using a diode detector variably coupled to an audioamplifier, as by means of a volume control potentiometer resistor, andspecifically in cases involving a combined thermionic diode-triode orpentode tube used as a second 10 detector-first audio amplifier in asuperheterodyne, considerable trouble has been encountered because ofoverloading of the grid circuit of the audio amplifier when the volumecontrol was advanced to maximum position. The grid of the audioamplifier usually has been operated with a fixed bias obtained from thevoltage drop in a self bias resistor in the cathode lead, the resistorbeing by-passed by a large value capacitor, usually a 10 mi.electrolytic type. The grid of the amplifier has commonly been connectedto the coupling potentiometer in the diode circuit through a stoppingcondenser to isolate direct current or rectified carrier so that onlythe audio frequency component of the detected signal is impressed on thegrid.

It has usually been desirable, from the standpoint of cost and size ofapparatus, to arrange the tubes and design a circuit that will givesatisfactory performance with the above mentioned detector-amplifiertube feeding directly into the grid circuit of a power output tube. Thishas occasioned the use of a duplex diode high mu triode tube, such as anRCA-'75, and which operates at a very low value of substantially fixedgrid bias, say negative 1 volts. When a strong signal was tuned in onthe receiver a d. 0. (direct current) voltage component from therectified signal of the order of 20 volts has ordinarily appeared in thedetector circuit across the resistors in circuit with the diode,available for automatic volume control of the tubes ahead. If thissignal is modulated 50%, an audio signal voltage of the order of 10volts a. c. (alternating current) is available in the diode circuit.Under these conditions, when the audio volume control has been turned tomaximum, all, or a substantial amount, of this audio voltage was appliedto the grid of the high mu a. f. (audio frequency) tube, it is seen thatvery serious overloading and distortion has resulted because of the factthat the audio voltage applied to the grid was several times the fixedbias from the usual self bias resistor. Not only distortion in the gridcircuit of the tube has taken r place but also distortion in the diodecircuit has occurred because of the grid drawing current from the diodecircuit on the positive half of each cycle of the signal.

It is well recognized that what is known as delayed detection gives acertain amount of suppression of background noise from various. sourcessuch as thermal agitation of electrons, antenna pickup, etc., that is,quiet tuning between stations. This delayed detection has ordinarilybeen obtained by connecting the diode circuit return so as to obtain asmall threshold negative bias on the diode plate from the drop across aself bias resistor in thecathode lead of an associated amplifier.Delayed detection gives some distortion on certain weak signals and willnot detect still weaker signals, but it is widely used on automobileradio receivers because of simplicity and low cost. Byreason of thenovel arrangement and connections in my improved circuit, to behereinafter described, the effect of electron flow in a portion of mycircuit is employed in producing noise suppression by setting up athreshold bias without the use of additional elements and cost ofapparatus- In accordance with prior practice, noise suppression has beenobtained by means of a complicated network including a self biasresistor. .p

An object of my inventionis to provide a simplified and inexpensivevolume control in a thermionic detector-amplifier circuit wherebydistortion arising from the above mentioned causes is eliminated withoutincreasing materially the cost and complication of partsin the receiver.

A further object of my invention is to provide a volume control systemfor a high amplification type of thermionic device or tube in an audioamplifier wherein grid bias may be varied throughout a wide workingrange to prevent overloading and without introducing distortion.

A still further object of my invention is to provide simplifiedautomatic noise suppression circuit for giving silent tuning betweenstations.

The novel 'featureswhich I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from-the following description of a specificembodiment when read in connection with the accompanying drawing, likereference characters referring to similar parts in the differentfigures, and in which:

, Figure 1- is a schematic diagram of a portion of a superheterodynereceiving system embodying a preferred modification of my invention,

Fig. 2 is a circuit diagram of a circuit that I have been using in radioreceivers,

Fig, 3 shows a characteristic curve of the improvide circuit that willserve to illustrate my invention, and

Fig. 4 is a schematic diagram illustrating a laboratory circuit that wasused in obtaining the characteristic curve of Fig. 3.

In accomplishing the first named object of my invention it is desirablethat the gain of the amplifier section of the detector-amplifier ordiode-triode tube not be reduced, otherwise this would necessitateincreasing the cost and/ or complication of the apparatus as by theaddition of another tube and accompanying circuit elements. In receiversfor motor driven vehicles it is desirable to have maximum amplificationwith a minimum number of tubesand accessories in order to reduce batterydrain and for the sake of compactness, etc. While the practice has beento avoid variable bias when using a volume control with a high mu tubeas an audio amplifier because of serious distortion, variable bias hasbeen used with certain low mu tubes, e. g. the

: RCA-85 type, without substantial distortion, since it has thecharacteristic thatv a substantial bias increase does not cut off theplate-current, in other words, it has a low rate of change of mutualconductance with grid bias change.

Referring to Fig. 2 of the drawing, I have illustrated a circuit that Ihave employed in radio circuits and which follows some of the teachingsof K. A. Chittick, patent application Serial No. 678,153, filed June 29,1933, now Patent No. 2,047,003, of same assignee. The control grid of alow mu tube 2 of the RCA-85 type is directly and variably connected to apotentiometer resistor 21 in circuit with the diodes l5 and 16. It willbe seen that, by reason of the direct connection, as the contact on thepotentiometer 2| isadvanced toward the-maximum position, not only the a.f. voltage but the d. c. or rectified carrier component are.simultaneously increased on the grid. In order to. limit the platecurrent for the condition where the grid has impressed on itinsufficient negative diode bias, a coupling resistor l2 of around25,000;0hms, a high value relative to the anode resistance, is used inthe plate voltage supply to the. anode. The plate of the tube. 2, is.connected through a stopping condenser 9 to a high mu power output tubeI0. While this circuit has performed in a. satisfactory manner withmanually variable bias, it was found necessary to use a fairly highstep-up transi former, e. g. a ratio of 1:4 for practical purposes, inan attempt to make up for the low gain of'the low mu type tube 2. Fromthe standpoint of fidelity of reproduction, cost, reduction in size andsimplification of apparatus, it would be preferable, however, tousestraight resistance coupling between tube 2 andthe output tube 10 butthe sacrifice in gain, with; a low mu tube 2 would be too great, Toobtain comparablefidelity with transformer coupling as with resistance,it

; would be necessary to have a high impedance primary, meaning a largenumber of turns, and several times as many secondary turns, avtransformer of excessively large proportions. To obtain the same gainwith a transformer coupled low mu tube as with resistance coupled highmu tube would require ashigh as 1:10 step-up ratio. Such a transformerwould be impractical if designed to give satisfactory operation.

In accordance with my invention 1' have found that a highmu tube may beoperated satisfactorily without appreciable distortion throughout arelatively wide range of bias potentials on the grid when certainconnections to the screen grid, hereinafter described, are made inaccordance with the teachings of A. H. Turner, as disclosed and claimedin application Serial No. 482,193, filed September 16, 1930, now PatentNo. 2,063,143, of same assignee. Turner discloses that in controllingvolume in a radio frequency amplifier the cut-off point of screen gridtubes may be extended very substantially as the negative bias on thecontrol grids is increased if the positive potentials on the screengrids are simultaneously increased in a certain manner.

Referring to Fig. l of the drawing, I have illustrated a portion of thecircuit of a superheterodyne receiver wherein an intermediate frequencyamplifier for modulated carrier currents, including a pentode tube lwith a tuned i. f. (intermediate frequency) input transformer secondary3 and step-down output i. f. transformer 4, feeds a detector diode I5 ina combined second detector and first a. f. amplifier tube 8. Thedetector and amplifier may be in separate tubes if desirable. The outputelectrode or anode 34 of the tube 8 is resistance coupled throughresistor 5, stopping condenser 9 and leak resistor I to a high mu poweroutput tube In. A loudspeaker I I is shown connected through atransformer 13 to the output of the power stage.

It is advantageous to connect the second diode plate Hi to groundinstead of tying it to the other diode plate [5 in accordance with usualpractice. The effect of this connection is to more completely shield thesignal diode 15 from the control grid 21 and thereby make it possible toobtain a lower minimum volume. It has heretofore been found in high gainsets, and with strong impressed signals, that there is suflicientcapacity coupling between the diode and control grid to pass sufficientsignal voltage to prevent obtaining desired low volume with minimumsetting of the volume control on the diode.

Referring more in detail to my improved circuit, signal voltage from thedriving transformerv 4 is impressed between the diode plate electrode [5and the cathode or source of electrons ll of the tube 8, and theresulting detected signal voltage appears across resistors l9 and 2|.The entire drop in voltage of the d. c. or rectified carrier componentof the detected signal is used for biasing the modulated carrierfrequency amplifier tubes, not shown, preceding the i. f. tube I forautomatic volume control purposes, as indicated by the arrow at AVC. Aportion of the d. c, voltage drop is used for automatically controllingthe bias on the grid of the i. f. amplifier I, as indicated by theconnection to point 23.

The control grid 21 of the diode-triode tube 8 is variably connected toresistor 2|, used as a potentiometer for controlling the audio voltageand thereby the volume output from the receiver. As the. slider on thepotentiometer 2]. is advanced the d. c. bias increases as a function ofthe increased a. f. For the maximum positions, as shown, the a. f. aswell as the d. 0. component is at the maximum desired. It will,therefore, be impossible to overload the grid, that is for the positiveaudio swings on the grid to exceed the negative d. c. bias and draw gridcurrent. In one sense this is an improvement over Heising 1,687,245,broadly considered, who provided diodes for increasing the negative griduse of a resistor 29 of the order of one megohm 'in series in the d. 0.supply circuit of a screen grid 3| which may also be considered a firstanode or an output electrode. The screen grid is by-passed togroundthrough a condenser v33 of the order of one-tenth mfd. Thisresistor 29 is of much higher value than ordinarily used for filteringpurposes, and, in accordance with the teachings of Turner, abovereferred to, the screen grid is efiectively floating for d. c. po-

l5 tentials and assumes diflerent values depending upon the d. c. biason the control grid. As the control grid becomes more negative, therebyreducing the space current, less current flows in the screen gridcircuit with the result that the 0 screen grid potential becomesincreasingly positive, approaching the anode potential, a differentialeffect with respect to the grid voltage variation. j

A suppressor grid 32 makes the screen grid 5 current a constant quantityfor different tubes to impart a definite characteristic to screen gridtubes in production. Otherwise, secondary emission, an uncertainquantity, would upset conditions to such an extent as to make it dif- 0ficult' to use the above arrangement for production. For best operation,the values of the anode resistors 5 and screen resistor 29 areimportant.In the first place the value of the plate resist- 5 ance load determinesthe gain, around 150,000 ohms, by way of example, giving a desired highgain. In an actual receiver that I have built in accordance with myinvention, the plate load resistance 5 has. been divided into two series"40 sections, So being around 56,000 ohms and 5b being'of the order of100,000 ohms. Section 5b isby-passed to ground with a .05 mf. condenser30 whereby the combination of resistor 5b and condenser 30 functions toboost low a. f. frequencieslaswell as provide additional filtering.

quencies. The-opposite eifect takes place in the case of high audiofrequencies. By way of additional example stopping condenser 9 wasaround .01 mid. and resistor 1 around 560,000 ohms.

With the desired gain obtained, the value of the 5 screen resistor29 isadjusted to give minimum distortion. I have found a value of l megohmfor resistor 29 to be-satisfactory with a load of the above amount inthe plate circuit. The above values are given merely by way ofillustration and should not be construed as limiting my invention. I

In Fig. 3 I have shown a static curve A and dynamic curves B of thecharacteristics of iny circuit. In Fig. 4 I have illustratedthefundamental circuit used in the laboratory for deriving the abovecurves. In deriving curve A, the control grid was adjusted to knownvalues of negative bias voltage on a C battery source, 0 and platecurrent in milliamperes was read on a meter M in the plate circuit, theplate volt- 7 age from battery B being 250 volts. Resistors 5 and 29were 150,000 .ohms and 1,000,000 ohms, respectively. The tube employedwas very high .mu. in eharacteristic, the amplification factor condenser33 in Fig. 1. bias of 1 grid 21 was set at, say 5 volts; then the gridtakes place at no signal bias.

sistance about 800,000 ohms.

While curve A illustrates the static or d. c. rela tion between the gridbias and plate current curves B represent the dynamic or a. 0. operationat two values of grid bias selected at random, namely, -5 and. 15 volts,and on the basis of corresponding fixed screen potentials, which ocourin practice by reason of the a. f. by-pass For deriving curves the thecorresponding screen voltage was deter- -mined; next, since in actual a.i. operation the screen voltage remains constant for a given signal gridbias, for reasons just given, the screen 3| was directly connected to apoint on the B battery of the same voltage as that determined;

' then the d. c. bias on the signal grid was varied to obtaincorresponding values of plate current. The resulting dynamic curves Bshow that "the tube can be worked at the point of the characteristiccurve giving small distortion and at the same time permit the grid biasto be varied over a much wider range than normal.

While in accordance with my invention it is desirable to employ apentode having a screen potential that varies by reason of poorregulation in the screen circuit, it would be possible to operate a highmu triode with variable control grid bias as an a. f. amplifier,provided the anode is connected through resistance in the anode circuitthat is several times as high as the internal plate resistance. A tube,for example, having the characteristics of an RCA-75 type tube can beconnected through a million ohm resistor to a voltage source of around1000 volts, an excessively high and impractical value. It will be seenthat, as the control grid negative bias is increased, the plate currentdecreases, with the result that theimpressed plate voltage rises byreason ofthe decreased voltage 'drop through the high anode resistance,thereby substantially delaying the point .of cut-oft. While such amodification can be used with a tube designed to withstand theabnormalvoltages it will be quite obvious that my preferred modificationis free of the disadvantages inherent in the proposed use of the high mutriode.

, Another important aspect of my novel detectoramplifier circuit is thata substantial amount of automatic noise suppression occurs in thearrangement employed, by reason of a delay bias on the detector. Thisaction is desirable from the standpoint of silenttuning betweenstations. Al-

though noise suppression from delayed detec-" tion? bias, of an amountemployed in compromise with fidelity and sensitivity of detection, isinsufiicient tov give complete silent tuning between 3 stations, theamount generally employed, and as obtained with my system, is enough tomaterially reduce the background noise when the set is tuned off acarrier wave. I attribute this action to the fact that the tube 8employed by me has a fine mesh control grid closely spaced to thecathode, and consequently a substantial electron fiow to I desire todesignate this eifect as residual grid current. Such grid current occursby reason of the Edison effect or contact potential, initial electronvelocity, etc.

The resultant residual current continues to flow in the grid circuitwhen the grid has applied voltage values up to about 1 volt negative,depending upon the tube construction, the amount varying with difierenttubes tested. Of the, tubes .being around 800 and the interna platei l0.ample, due to its own grid current.

I have used the average voltage involved was around tths of a volt, Forsome purposes it may be desirableto-employ tubes so designed as to givemuch higher residual grid voltage. Heretofore tubes have not beendesigned with theintention of causing grid current and many haveconsideredsuchcurrent undesirable. With sufficient resistance in thegrid circuit, the grid biases ,itself to negative ths of a volt, intheabove-ex- Since-the grid of the tube is connected directly to thediode circuit, the ths volt is developed across the diode resistancenetwork when a substantial amount of resistance is included in circuitby the ,15 volume control, and is in such a direction as to therebyreducing noise output from the receiver.

Of course when the volume control is set at or near minimum, as inthereception of strong signals, the necessary voltage from the gridcurrent for cutting off the diode is not developed for noisesuppression, but in this case the noise islow and unobjectionable. Insome cases the grid may .be connected to a-fixed point on the dioderesistor and not related to the volume controlthat .maybe used elsewherein the circuit. Since the tubes controlled by automatic volume controlare connected to the resistor in the diode circuit, the suppressionvoltage will increase the negative bias on the controlled tubes overwhat it would otherwise be when tuning between stations, or in otherconditions'of signal level below-a certain ,10

.' minimum value. v

It will'be understood that-there'occurs also a residual electron flowfrom cathode to diode plate of the detector under the condition of noim- 40 pressed potentials up to a certain amount, -bediode circuit. Thisbias action by the diode perse should not be confused with delayeddetection. The-application of negativepotential to the diode, from anexternal source as from the grid asabove described, greater than thediode voltage, cuts off electron flow to the diode plate so thatdetection does not occur until an impressed signal is of sufiicientmagnitude to overcome this delay or threshold value. From one point ofview the diode is biased beyond cut-off by the grid current.

In accordance with my invention the residual grid voltage exceeds theresidual diode voltage by a safe margin. In working with detector diode-b'ias on low mu a. f. amplifiers in high fidelity reundesirable, Mr.George L'. Beers found that in same rare cases in production, grids oflow mu tubes drew current at a higher negative voltage than did thediodes used. Accordingly he took steps to insure against imposing adelay bias on the diode in such cases by preventing grid current flow asby means of a negative C bias on the grid. I

However, I have found the grid current to'be useful in setting upa-threshold bias for noise suppression without the use of addedapparatus, 3 cost, and complications.

' circuit.

the amplifier controlled.

arter herent or residual grid current flow, very.-materially aidin-noise suppression when the receiver .is tuned oft carrier,' or" whena carrier is below a certainlowrninimum level. Either of .thesetwo-actions {nay be use'dsingly. While I have shown the detector a's biasedthrough the diode resistor I by the grid eta high mu amplifier. fornoise suppression it. isp o'ssiblei that some electrode other than the.control grid can'lbe "especially designed for use as a source of'delaybias on the diode, in

. which case it would be unnecessary to'use a high mu tube. 1

What I claim as my invention is:' I. In a radio receiving system a firstthermionic amplifier, a. detector, and second thermionic amplifier,means for impressing modulated carrier current amplified by said firstamplifier onsaid .'detec'tor,".a resistance network in circuit with saiddetector for'developing, therein the modulation andrectified carriercomponents,

'a' conductive connection from the g'rid of said second amplifier to, aportion'of said resistance network, said grid fbeing. characterized bythe ,fac't'that a substantial amount of residual grid current 1' fiows'with impressed potentials below "a certainlevel, whereby, lbyreasori'ofsaid conductive connection 'a potential bi'as'is developed in saidresistance network by said grid current, said bias rendering .said"detector inoperative foigimpressed signal orfnoise energy below acertain predetermined minimum level.

l '2. The system'jas set 'Iorth'in'thejnext precedmgclalmtharacterized'in that connections are provided tosaidnetwork toderivea potential'for automatic'volu'm-e ,control of said firstamplifier, whereby said biasdevelope'd by saidresidual grid currentcauses 'a decrease in amplification of 3."In a radio receiving systema'first thermioriicarhplifier, a detector, and second ther- ,mion1camplifier, means for impressing modulated carrier current amplified bysaid first am plifier 1011 said detector, a resistance network incircuit .withtsaid detector for developing therein the modulation andrectified carrier components,

"a conductive connection from the grid of said "second amplifier toaportionfof said resistance network, said-gridbeingcharacterized by thefact that a substantial amount of residual gridcurrent flows withimpressed potentials below a cer- --tain level,connectionstfromsaid'first amplifier to said ;network to derive .a potential forautomatic 7 volume control thereof, whereby voltage e e o edzby s d reidua g d u en i s networkvcauses. a decrease in .amplification ofceivers, where delayed detection was considered first amplifier forinput energy below a predetermined minimum level; i

4. In a radio receiving'systemi a thermionic tube including a detectore1ectrode,,an additional electrode injsai'd tube, a source. of"elecsaidresistance network by residual current fiow f to said detector"electrode; therebypreventing "electron fiow to saiddetectorelectrodefor 1m pressed potentials below the threshold valueestablished by the residual current flow to said additional electrode.

5. The invention as set forth in the next preceding claim characterizedin that said additional electrode is the grid of a high mu thermionicamplifier.

6. In a modulated carrier signal receiving system, a thermionic diodedetector having a plate, a thermionic amplifier having a grid, avariable resistance network common to said detector and amplifier andconstituting a direct current connection between said plate and grid forimpressing detected signal components on said grid to adjustably changethe grid bias as a function of the changed audio component, said gridbeing characterized by the flow of a residual electron current throughsaid resistance network in the absence of signals above a minimum level,for biasing ofi said detector.

7. In a receiving system, a diode having a plate, and a high muamplifier having a grid, a resistance network providing a direct currentconnection between the plate and said grid, means for variablyimpressing diode bias on said grid from said network, said grid beingcharacterized by residual electron flow thereto and through said networkin the absence of impressed potentials above a certain level, theresidual grid current being greater than the residual diode current,said grid being so connected to said network as to negatively bias offsaid diode in the absence of signals above a certain minimum level.

8. In a radio receiving system a diode detector and a thermionicamplifier, means for impressing modulated carrier signals on saiddetector, a resistance potentiometer in circuit with said detector fordeveloping demodulated signal energy components therein, a directconductive connection between the grid of said amplifier and a variabletap on said potentiometer for adjustable diode biasing, said grid beingcharacterized by a residual electron current flow at values of diodebias below a certain minimum level, said grid current exceeding theinherent residual flow in said detector, said grid current flow causinga potential in said potentiometer for biasing ofi said detector for theprevention of residual elec-' tron fiow in said detector.

9. In a radio receiving system, a detector, means for producing noisesuppression comprising a thermionic device for generating a voltage tobias said detector with a threshold value beyond cut-ofi for signalsbelow a certain minimum voltage level, connections from said detector tosaid device for rendering the latter ineffective to generate said biaswhen receiving signals substantially above said level.

10. In a radio receiving system, an amplifier, a detector for signalsfrom said amplifier, means for producing noise suppression comprising athermionic device for generating a negative bias by the Edison effect,connections from said device to said amplifier for impressing saidnegative bias on the latter in the absence of signal energy above acertain minimum level, connections from said detector to said device forrendering the device inelfective for the production of said bias whenreceiving signals substantially above said level.

LESTER T. FOWLER.

CERTIFICATE -OF CORRECTION,

Patent No, 2,075,611,% March 56, 1957,

LESTER T, FOWLER,

It is hereby certified that error appears Y in the printed specificationof the above numbered patent requiring correction as follows: Page 2,second line 7, for the patent number "2,065,119" read 2.,O65,l l5; page5,

column,

and that the said Letsecond column, line 10, for "curves" read curves B;ters Patent should be read with these corrections therein that the samemay conform to the record of the case in the Patent Officeo Signed andsealed this (th day of December, A. De 1957,

Henry Van Arsdal'e,

(Seal) Acting Commissioner of Patents 9

